1. Field of the Invention
The present invention is related to a circuit and system for correcting for the doppler frequency effect of a moving airborne station relative to a fixed ground station. More particularly, the present invention relates to a circuit for automatically compensating for the doppler frequency effect between two stations, either or both of which may be moving relative to the other and providing a highly precise real time compensated R.F. carrier signal for uplink transmission to the airborne station.
2. Description of the Prior Art
Heretofore, systems and circuits were known for grossly compensating for the doppler frequency effect that occurs between two stations which are moving relative to each other. Such prior art systems are classified as coherent and non-coherent carrier transmission systems. In the non-coherent carrier transmission system, the receiving station does not require knowledge of the exact frequency or phase of the received signal to demodulate the data properly. Such systems are incapable of providing coherent doppler compensation which is provided by the present invention.
In contrast thereto, in a coherent carrier transmission system, the airborne station is continuously transmitting at a fixed carrier frequency which enables the ground station to coherently lock onto the carrier signal and phase which is arriving at the ground station at a doppler shifted frequency. Since the ground station is locked onto the received down link doppler-shifted carrier frequency, it may coherently and continuously compensate for the doppler effect and transmit a pre-compensated uplink R.F. carrier frequency signal at a frequency which is equal to the predetermined carrier frequency required for the airborne station, thus substantially reducing the time for acquisition of the R.F. carrier frequency signal at the airborne station.
Heretofore, doppler frequency compensation circuits and systems were capable of producing doppler compensated R.F. carrier frequency signals to the uplink or airborne station, however, such systems only provided a gross estimate of the required R.F. carrier frequency signal and furthermore the compensation circuits had been subject to drift, aging and temperature variations which produce errors in excess of more than one hertz. When such carrier frequency errors occur, it is often necessary for the airborne station to perform a frequency search before it can lock onto the phase of the carrier frequency. Thus, if the R.F. carrier frequency transmitted to the airbone station is in error, the phase-locked loop has to modify the desired predetermined quiescent frequency of the airborne station demodulator's VCO in order to lock onto the received R.F. carrier frequency. Heretofore, the inaccuracies in the prior art doppler frequency compensation circuits and systems have required excessive time for acquisition on the part of the airborne station. While this condition may be acceptable for a single ground station transmitting to a single airborne station, it is extremely important for time division multiple access (TDMA) networks which employ a plurality of ground stations cooperating with a single preferred airborne station. When the data acquisition window must be lengthened to provide for frequency acquisition time, the data throughput to the airborne station is substantially diminished.
Accordingly, it would be extremely desirable to provide a highly accurate doppler frequency compensation system and circuit which does not require frequency search by the airborne station over a plurality of hertz of frequency uncertainty and which permits the airborne station to lock onto the uplink carrier with only a phase transient prior to carrier lock. Such a desirable system would be capable of obtaining frequency lock on the uplink R.F. carrier signal without a frequency search and with no cycle slipping, thus permitting extremely rapid acquisition of the carrier signal and operation at higher carrier frequencies than were heretofore obtainable.